(UP Big Boy) at Steamtown National Historic Site in Scranton, Lackawanna County, PA. The largest steam locomotive in the world. It was built by the American Locomotive Company (ALCO) in 1941 and is the only locomotive with the unique 4-8-8-4 wheel arrangement. This monster boasted 7,000 horsepower and could pull 100 cars across Wyoming at speeds of up to 60 mph. This massive engine with Tender weighs just over 600 tons!! 25 of these monsters were built for the Union Pacific. (N41 24.510 W75 40.300) Photo by Mark Comstock 10-17-98.